The sporulation of Saccharomyces cerevisiae is a relatively simple model system for the study of the control of a differentiation process which occurs after the cessation of proliferation. In particular, the formation of four haploid ascospores from a diploid cell provides an excellent opportunity to investigate the process of gametogenesis in a unicellular organism in a controlled environment and free from interactions of cells not undergoing specialization. The proposed research is centered on two major aspects of the control of the transition of a cell from the mitotic vegetative state to meiosis and sporulation. First, research is proposed to examine the primary control mechanisms which govern the entire transition process. The capacity of a diploid cell to sporulate depends on the presence of both sexual (mating type) alleles. Temperature sensitive mutants which cause a conditional conversion of the mating phenotype will be isolated to study the role of the balance between the two sexual phenotypes in the establishment and maintenance of meiosis and the maturation of spores. Biochemical and cytological tests will be applied to verify and characterize an apparent all-or-none mechanism whereby cells either complete sporulation or remain fixed in a vegetative state. The second major objective is to examine the degree to which the early part of the sporulation process, up to the time of meiotic DNA replication, is similar to the vegetative mitotic cell cycle which preceeded it. An initial aim is to determine if enzymes whose role is common to both growth and differentiation are actually synthesized again during sporulation. A second question is whether those enzymes which are synthesized during sporulation are expressed under the same control mechanisms which govern gene expression in the vegetative state. A number of enzymes involved in the utilization of acetate will be measured in yeast both grown and sporulated in acetate media to show whether the order of appearance and mode of synthesis (periodic or continuous) is unchanged during differentiation.